Silver cadmium oxide electrical contact material and method of making

ABSTRACT

An improved silver cadmium oxide electrical contact material which is pore free and malleable and which has an effectively uniform cadmium oxide content throughout the thickness thereof is shown to be made by metallurgically bonding an intermediate layer of silver cadmium alloy of selected cadmium content between other layers of silver cadmium alloy of relatively lower cadmium content to form a composite metal material and by heating this composite material in an oxygen-containing atmosphere for diffusing cadmium from said intermediate metal layer into said other metal layers to effectively equalize the cadmium distribution throughout all of the metal layers and for oxidizing the cadmium within the composite material to form an electrical contact material having cadmium oxide particles uniformly distributed within a silver matrix.

United States Patent Jost [ Apr. 30, 1974 1 SILVER CADMIUM OXIDE ELECTRICAL CONTACT MATERIAL AND METHOD OF MAKING [75] Inventor: Ernest M. Jost, Plainville, Mass.

[73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

[22] Filed: Sept. 11, 1972 [21] Appl. No.: 288,236

[52] US. Cl. 75/173 A, 29/195, 29/199 [51] Int. Cl. C22c 5/00 [58] Field of Search 75/173 A; 29/195, 199

[56] References Cited UNITED STATES PATENTS 3,666,428 5/1972 Haarbye 29/195 3,674,446 7/1972 Haarbye et al. 29/195 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-15. L. Weise Attorney, Agent, or FirmHarold Levine; John A. Haug; James P. McAndrews [57] ABSTRACT An improved silver cadmium oxide electrical contact material which is pore free and malleable and which has an effectively uniform cadmium oxide content throughout the thickness thereof is shown to be made by metallurgically bonding an intermediate layer of silver cadmium alloy of selected cadmium content between other layers of silver cadmium alloy of relatively lower cadmium content to form a composite metal material and by heating this composite material in an Oxygen-containing atmosphere for diffusing cadmium from said intermediate metal layer into said other metal layers to effectively equalize the cadmium distribution throughout all of the metal layers and for oxidizing the cadmium within the composite material to form an electrical contact material having cadmium oxide particles uniformly distributed within a silver matrix.

4 Claims, 5 Drawing Figures 1 SILVER CADMIUM OXIDE ELECTRICAL CONTACT MATERIAL AND METHOD OF MAKING In a conventional process for manufacturing silver cadmium oxide electrical contact materials in a very economical way, a strip of silver cadmium alloy material of selected-thickness is heated in an oxygen atmosphere for internally oxidizing the cadmium portion of the alloy in situ within the material, thereby to convert the metal alloy to metal-metal oxide alloy having a very large number of-very small cadmium oxide particles distributed throughout a silver matrix. When electrical contact materials are formed of material made in this manner by blanking contacts from the internally oxidized strip material, it is found that this metal-metal oxide composition is quite malleable for permitting contacts to be easily formed from the material and the contacts display excellent resistance to are erosion and resistance to contact sticking during initial use of the electrical contacts. However, it is found that some are erosion does occur during use of these electrical contacts and most important, it is found that, as the surfaces of the contacts are eroded during use, the resistance to contact sticking rapidly decreases due to the existence of what is called a depletion zone at the center of the contact where the cadmium oxide content of the contact material is found to be relatively low. As a result, the frequency of contact failure through contact sticking tends to markedly increase. Other processes for making the desired metal-metal oxide contact mate rials are known. However, these other processes tend to form electrical contact materials which are somewhat porous and which are generally somewhatless malleable or even brittle so that the materials have to be treated in other ways before forming contacts from these other materials.

In this invention, it has been found that the depletion zone located at the center of electrical contacts formed of the internally oxidized metal-metal oxide contacts above described is largely due to diffusion of cadmium from the center of the silver cadmium alloy strip while the strip is being heated for internal oxidization thereof. It has then been discovered that this depletion zone can be effectively eliminated by pressure-bonding a layer of silver cadmium alloy of selected, relatively high cadmium content between other layers of silver cadmium alloy of relatively lower silver cadmium content to form a composite silver cadmium alloy material and by heating the composite material in an oxygencontaining atmosphere for internally oxidizing the cadmium content of the composite material, the characteristics of the various silver cadmium alloy layers being selected with respect to the time and temperature parameters of the internal oxidizing process such that diffusion of cadmium outwardly from the center of the composite material tends to equalize cadmium content throughout the material as the cadmium material is being oxidized. In this way, the resulting silver cadmium oxide strip material is formed effectively free of the depletionzone found in the prior art material but continues to achieve the desirable characteristics of malleability, small cadmium oxide particle size and freedom from pores which had been achieved in the internally oxidized metal-metal oxide material of the prior art.

It is an object of this invention to provide novel and improved silver cadmium oxide electrical contact material; to provide such contact material which is malleable and pore free and which has an effectively uniform cadmium oxide content throughout the thickness of the material; to provide novel and improved processes for making such contact material; and to provide such processes which are easily and economically performed.

Other objects, advantages and details of the electrical contact materials and processes of this invention appear in the following detailed description of preferred embodiments of the invention, the description referring to the drawings in which:

FIG. 1 is a transverse section view illustrating characteristics of internally oxidized silver cadmium oxide materials of the prior art, the section lines being omitted from this view to permit illustration of the characteristics of the prior art material by use of stippling;

FIG. 2 is a diagrammatic view illustrating the process of this invention;

FIG. 3 is a transverse section view to enlarged scale along line 33 of FIG. 2 illustrating an intermediate product formed during performance of the process of this invention, the section lines being omitted from this view to permit illustration of the characteristics of the intermediate product by use of stippling;

FIG. 4 is a transverse section view along line 4-4 of FIG. 2 illustrating the product of the process of this invention, the section lines being omitted from this view to permit illustration of the characteristics of the product of this invention by use of stippling.

In a prior art process for producing silver cadmium oxide electrical contact material in strip form, a strip of silver cadmium alloy material, with or without small quantities of grain refining additives such as cobalt, sodium or the like therein, is pressure-bonded to a relatively thinner layer of fine silver material. This composite material is then placed in an oxygen-containing atmosphere and is heated at a selected temperature of about l,500 F. which has been found to be effective for oxidizing the cadmium content of the silver cadmium alloy layer of the composite at a suitable rate without significantly altering the characteristics of the silver material in either the silver cadmium or fine silver layers of the composite. In this way, the cadmium content of the silver cadmium alloy layer of the composite is converted to cadmium oxide, this oxide material being produced in the form of a multiplicity of small cadmium oxide particles which are dispersed throughout a silver matrix in what had previously been the silver cadmium layer of the composite. This layer of metal-metal oxide material has the layer of fine silver secured thereto so that, when electrical contacts are blanked from the resulting composite, the fine silver material serves as means for soldering the contacts to contact arms and the like.

In the electrical contact industry, it had been conventional to produce such internally oxidized contact material with cadmium oxide contents ranging from about 2.5 to 20.0 percent by weight of the metal-metal oxide layer of the composite, the higher cadmium oxide contents providing the contact material with greater resistance to contact sticking or welding and the lower cadmium oxide contents providing the materials with relatively better electrical conductivity properties, thereby to meet the needs of a variety of electrical contact applications.

However, it had been found that while the electrical contact strip material formed by this prior art process was desirably free of pores and displayed satisfactory malleability permitting electrical contacts to be readily blanked from the strip material, the contact material tended to have a relatively very low cadmium oxide content at the center of the contact material. That is, as is shown in FIG. 1, the prior art contact material incorporating the metal-metal oxide layer 12 and the fine silver backing layer 14 had a cadmium oxide content, indicated by stippling 16 in FIG. 1, which was much greater at the surfaces of the metal-metal oxide layer 12 than at the core of this layer. As a result, when the contact material was provided with the surface cadmium oxide necessary for meeting the needs of a specific contact application, the contact material displayed excellent arc erosion properties and resistance to contact sticking for a substantial period of time. However, as the surface of this contact material was gradually eroded during use of the contact exposing the cadmium oxide depletion zone at the center of the contact, the tendency of the contact material to stick or weld to a mating contact rapidly increased with a corresponding increase in the frequency of contact failure through contact sticking.

In accordance with this invention, as illustrated in FIGS. 2-4, a strip 18 of silver cadmium alloy of relatively high cadmium content is advanced from a pay-off reel 20 between a pair of pressure-squeezing rolls 22 while strips 24 of silver cadmum alloy of relatively lower cadmium content are fed from pay-off reels 26 to pass between the pressure rolls 22 at either side of the strip 18 and while a thin strip 28 of fine silver material is fed from the pay-off reel 30 along one side of the three silver cadmium alloy strip materials, these metal strips 18, 24, 24 and 28 being squeezed between the rolls 22 for metallurgically bonding the strips together to form a composite metal material indicated at 32 in FIG. 1. As will be understood, the strip materials 18 and 24 can also incorporate small quantities of grain refining constituents such as cobalt or sodium or the like therein within the scope of this invention. Further, the described strip materials can also be bonded together in the described sequence by any conventional means other than strip roll bonding within the scope of this invention.

In this arrangement, the composite material 32 as illustrated in FIG. 3 is shown to comprise metal alloy layers 18, 24, 24 and 28 metallurgically bonded together substantially throughout the interfaces between the layers, the layers 18 and 24 having relative cadmium contents (the higher cadmium content being illustrated in FIG. 3 by darker stippling) as shown in FIG. 3. In accordance with this invention, the composite material 32 is then advanced into a suitable retort 34 (see FIG. 1) filled with an oxygen-containing atmosphere 36 and is heated to a temperature of from 1,400 to 1,600 F., this heating being diagrammatically illustrated by the heating coils 38 in FIG. I. In this arrangement, the cadmium contents of the layers 24 of the composite material 32 tends to be diffused outwardly toward the outer surface of the composite material but, at the same time, the relatively higher cadmium content of the alloy layer I8 is also diffused outwardly from the surface portions of the layer 18 into the adjacent portions of the alloy layers 24 and is diffused outwardly from the center of the alloy layer 18 to the surfaces of that layer, thereby tending to equalize the distribution of the cadmium material in the composite 32 throughout the alloy layers 24 and 18 of the composite. Concomitantly, of course, the cadmium content of these alloy materials is oxidized in situ within the composite 32 forming the contact material 40 of this invention which is coiled on the take-up reel 41, this improved contact material having the desired multiplicity of very small cadmium oxide particles dispersed in a silver matrix. Thus, as is illustrated in FIG. 4, the resulting material 40 is found to embody cadmium oxide particles indicated by the stippling 42 in FIG. 4 which are distributed in an effectively uniform disposition in the silver matrix 44 throughout each of the material layers 18 and 24 in the contact material.

In this regard, it will be understood that performance of the process of this invention first requires that the silver cadmium alloy provided in the metal strip material 18 be selected so that, when the composite material 32 is heated for the internal oxidizing thereof as above described, the temperature to which the layer 18 of the composite is heated during said internal oxidation preferably does not exceed the melting temperature of the selected alloy and does not exceed the boiling temperature of the alloy. Accordingly, where commercially practical temperatures utilized in such internal oxidation are in the range from about 1,400 to 1,600 F., the silver cadmium alloy utilized in the metal strip material 18 preferably has a cadmium content less than about 28 percent by weight of the selected silver cadmium alloy. The maximum cadmium contents of the silver cadmium alloys embodied in the metal strips 24 are of course relatively lower as above described.

Further, where the intent of the present invention is to achieve substantially uniform cadmium oxide distribution through the metal-metal oxide layers of contact material 40 provided by this invention, the cadmium contents of the alloys embodied in the strip materials 18 and 24 are selected with respect to the desired cadmium oxide content to be provided in the contact material 40, with respect to the rate of diffusion of cadmium from areas of greater concentration to areas of lesser concentration during the internal oxidizing process step, and with respect to the temperature and the duration of the heat treatment required for internally oxidizing the cadmium contents of these alloys. Typically for example, effective equalization of the cadmium oxide contents is achieved throughout the metalmetal oxide layers of the contact material 40 where the strip materials 24 comprise about one-quarter of the total thickness of the silver cadmium portion of the composite material 32, where the cadmium contents of the strip materials 24 are about two-thirds of the content desired in the contact material 40, and where the cadmium content of the strip material 18 is about twice that of the alloy in the strip materials 24, the total thickness of the composite material 32 being selected with respect to the optimum oxidizing temperature to achieve effectively uniform cadmium distribution through diffusion in a time period coincident with completion of the oxidizing of the cadmium content of the strip materials.

For example, in a preferred embodiment of this invention, the strip materials 18 and 24 have a total thickness of about 0.500 inches before squeezebonding and are reduced about 60 percent during squeeze-bonding to have a thickness of about 0.200 inches in the composite material 32. The strip material 24 is provided with a cadmium content of about percent by weight while the cadmium content of the strip material 18 is about 20 percent by weight. The composite material 32 is then heated for about 20 hours at a temperature of l,500 F. in l atmosphere of air for substantially equalizing cadmium distribution in the layers l8 and 24 of the contact material 40 while substantially fully oxidizing this cadmium material. This resulting contact material is then found to be effectively free of any cadmium oxide depletion zone, is satisfactorily malleable and is free of pores and is adapted, after rolling to selected thickness if desired, to be readily blanked for forming electrical contacts of any conventional type.

As will be understood, the embodiments of the process and contact material described above have been l. A method for making silver cadmium oxide electrical contact material which is malleable and pore free and which is substantially free of a cadmium oxide depletion zone at the center of the material, said method comprising the steps of metallurgically bonding a first layer of silver cadmium alloy having a selected cadmium content between other layers of silver cadmium alloy of relatively lower cadmium content to form a composite material and heating said composite material in an oxygen-containing atmosphere for diffusing said cadmium from said first layer into said other layers to avoid development of a cadmium depletion zone in said layers during said heating and for oxidizing said cadmium within said layers.

2. A method as set forth in claim 1 wherein said layers of silver cadmium alloy are provided with selected cadmium contents and are heated for diffusing said cadmium for substantially equalizing the distribution of said cadmium throughout said layers and for oxidizing said cadmium within said layers.

3. A method as set forth in claim 1 wherein said first layer of silver cadmium alloy has a cadmium content comprising less than about 28 percent by weight of said alloy and wherein said composite material is heated at a temperature in the range from about 1,400 to l,600 F. for diffusing and oxidizing said cadmium.

4. A method as set forth in claim 1 wherein a layer of fine silver is metallurgically bonded to one of said other alloy layers to form an outer surface of said composite material prior to said heating thereof. 

2. A method as set forth in claim 1 wherein said layers of silver cadmium alloy are provided with selected cadmium contents and are heated for diffusing said cadmium for substantially equalizing the distribution of said cadmium throughout said layers and for oxidizing said cadmium within said layers.
 3. A method as set forth in claim 1 wherein said first layer of silver cadmium alloy has a cadmium content comprising less than about 28 percent by weight of said alloy and wherein said composite material is heated at a temperature in the range from about 1,400* to 1,600* F. for diffusing and oxidizing said cadmium.
 4. A method as set forth in claim 1 wherein a layer of fine silver is metallurgically bonded to one of said other alloy layers to form an outer surface of said composite material prior to said heating thereof. 